Drainage improvement in paper pulp suspensions containing lignin residues

ABSTRACT

DRAINAGE IS IMPROVED IN A CELLULOSIC PULP SUSPENSION CONTAINING SOLUBLE LIGNIN RESIDUES BY THE ADDITION TO THE SUSPENSION OF A HIGH MOLECULAR WEIGHT WATER-SOLUBLE ANIONIC POLYMERIC AND ALUM IN A QUANTITY GREATER THAN THAT NORMALLY USED.

United States Patent O 3,706,629 DRAINAGE IMPROVEMENT IN PAPER PULP SUS-PENSIONS CONTAINING LIGNIN RESIDUES Edgar E. Moore, Midland, and WilliamA. Foster, Mapleton, Mich., assignors to The Dow Chemical Company,Midland, Mich. No Drawing. Filed July 23, 1970, Ser. No. 57,782 Int. Cl.D21h 3/38, 3/18, 3/16 US. Cl. 162-168 13 Claims ABSTRACT OF THEDISCLOSURE Drainage is improved in a cellulosic pulp suspensioncontaining soluble lignin residues by the addition to the suspension ofa high molecular weight water-soluble anionic polymeric polyelectrolyteand alum in a quantity greater than that normally used.

BACKGROUND OF THE INVENTION The invention is an improvement in themanufacture of paper, paperboard, or other such cellulosic fiber sheetsgenerically referred to as paper from aqueous cellulosic fibersuspensions containing lignin residues, the so-called black liquorsolids.

The term black liquor solids is used herein to refer to lignin residuescharacteristically present in some papermaking systems. Black liquor isthe liquor containing residues formed by cleavage and solubilization oflignin during the cooking of pulpwood. In the neutral sulfitesemi-chemical process, the lignin residues in the black liquor areprimarily lignosulfonates. In the manufacture of corrugated medium fromneutral sulfite semi-chemical pulp, the black liquor solids are usuallypresent in concentrations of 25-100 percent based on the fiber content.In the manufacture of kraft linerboard, the black liquor solids arepresent in lower concentrations of the order of 2-10 percent of theweight of fiber. These solids are largely substituted sodium phenolates.The improvement concerns specifically the use in such suspensions of acombination of aluminum ion and an anionic watersoluble high molecularweight acrylic polymer.

In most papennaking systems, whether black liquor solids are present ornot, drainage of the pulp suspension in forming the paper web isaccomplished by filtration in the table roll or foil section of theFourdrinier wire using a relatively low pressure drop across the wetmat. This is followed by a vacuum filtration operation using a muchhigher pressure drop. The well-known Canadian Standard Freeness Testmeasurement provides a convenient and accurate indication of thedrainage rate for a particular suspension in the table roll or foilsection of the wire.

It is known to use alum and anionic water-soluble high molecular weightvinyl polymers to promote drainage and filler retention in paper systemswhich are free or essentially free of lignin residues, otherwise knownas black liquor solids. In paper systems containing black liquor solids,it is found that alum-polymer combinations show apparently favorableresults by the Canadian Standard Freeness Test, but actually hinderdrainage in practice and cannot be used. Apparently the alum-polymercombinations allow good drainage in the low pressure filtrationoperation, but hinder it in the vacuum filtration operation.

Cationic polymers such as N-aminoalkylated polyacrylamides, commonlyused in paper pulp systems not containing black liquor solids, can alsobe employed to accelerate drainage in paper systems containing .blackliquor solids, but the presence of these solids makes necessary the useof uneconomic amounts of polymer.

SUMMARY OF THE INVENTION It has now been found that drainage undervacuum filtration of aqueous cellulosic pulp suspensions containingsubstantial amounts of lignin residues is significantly improved whenthere is incorporated into such suspensions prior to formation of thepaper web about 0.02-0.1 percent based on the weight of dry cellulosicpulp of a Water-soluble anionic polymeric polyelectrolyte and aWater-soluble aluminum compound sufiicient to improve drainage of thepulp. The quantity of aluminum compound is in excess of that normallyused in these paper pulp suspensions. Illustrative anionicpolyelectrolytes are those of monomers containing acidic groups in theirmolecular structure such as acrylic acid, methacrylic acid, itaconicacid, fumaric acid, maleic acid styrenesulfonic acid, and other similarmonomers. .Also included are the water-soluble copolymers of these withother vinyl monomers such as acrylamide, methacrylamide, acrylonitrile,vinyl acetate, vinyl chloride, alkyl methacrylates, oxazolidinone,pyrrolidinone, and so on. Also useful for this use are water-solublehomopolymers and copolymers of the sulfoalkyl acrylates and carboxyalkylacrylate's such as sodium sulfoethyl acrylate and sodium carboxyalkylacrylate. Still other such polymers known to be useful anionicflocculants include carboxyalkyl cellulose derivatives and similarderivatives of other polysaccharides such as starch. Preferably, thepolymer is of relatively high molecular weight, i.e., at least about onemillion.

Preferred polymers are those of which the molecular structure isessentially composed of carbarnoylalkylene units of the formula R-OH,-('J- (IJONH:

and carboxyalkylene units of the formula wherein the carboxyalkyleneunits are at least about 20% and can constitute all of the total monomerunits present, and M is hydrogen or a water-soluble salt forming cationsuch as ammonium, alkaline earth metal, or alkali metal, preferablysodium, R is hydrogen or a methyl radical.

DETAILED DESCRIPTION The invention is applicable to any of the so-calledblack-liquor systems wherein significant quantities of lignin residuesare present in the pulp suspension. Specifically, these systems includekraft linerboard suspensions of unbleached kraft pulp where the blackliquor solids characteristically are present in the headbox pulpsuspensions in amounts of about 2-10 percent of the weight of fiber, andalso neutral sulfite semi-chemical pulp suspensions used for making thecorrugated paperboard which with the linerboard makes up the familiarsandwich construction of shipping containers, which suspensions in theheadbox may contain about 25-100 percent of black liquor solids orlignin residues based on the fiber content.

Kraft linerboard suspensions ordinarily are formed at about pH 5.5-7.5,usually at about pH 6-6.5, with about 0.5-1 percent fiber content andusually contain some rosin sizing in a quantity of about 0.1-0.5 percentbased on the weight of fiber. About 0.5-1 percent of alum based on fibercontent is conventionally used in such pulp suspensions. In the practiceof the present invention, polymer is also added as specified above andordinarily an additional quantity of alum as further defined later.

Neutral sulfite semi-chemical pulp suspensions normally are formed at ahigher pH, for example, about 7-9, but contain about the same proportionof fiber with a considerably higher proportion of lignin residues aspreviously described. These suspensions are usually made up with norosin sizing and no alum. In the process of this invention, however,both alum and polymer are added to such pulp suspensions.

The polymer used in the invention can be prepared by any conventionalprocess of polymerization or copolymerization. When bothcarbamoylalkylene and carboxyalkylene moieties are present in thepolymer structure, the polymerization in this case can be combined witha hydrolysis step wherein carbamoyl groups in a polymer structure areconverted to carboxyg-roups by reaction with a base such as sodiumhydroxide or sodium carbonate. Thus, a polyacrylamide orpolymethacrylamide can be converted to a polymer having the samestructure as that obtained by copolymerizing acrylamide and acrylic acidor the corresponding methacrylic monomers. As a practical matter, inorder to make a polymer having a relatively high proportion of acidmoieties in its structure, it is most convenient to copolymerize theamide and acid monomers in the desired proportion. In any case, polymersof high molecular weight are used, i.e., those having an averagemolecular weight of at least 1-10 million.

Preferably, such a polymer structure contains at least about 25 percentof carboxyalkylene moieties based on the total monomer units. With anincreasing proportion of carboxy groups, less additional alum or othersource of aluminum is required to obtain improved drainage. The carboxygroups in the polymer as used are ordinarily in the form of their sodiumsalt. The same is true of the acid groups in other polymers useful inthis invention.

Performance is also sensitive to other variables such as changes in pHand the amount of black liquor solids in the suspension. Generally, morealuminum compound is needed for optimum results at higher pH and higherconcentrations of black liquor solids. For example, in kraft linerboardsystems containing 2-5 percent of lignin residues, the minimum amount ofalum required for good results may increase from about 0.5-1 percent atpH 5 to about 5 percent at pH 7 while an increase in black liquor solidsto 8 or 10 percent may well require that the quantity of alum beincreased by at least half. In general, alum is used in this process ina quantity of about 0.2 to about 10 percent based on fiber content.Preferably, about 1-5 percent of alum or its equivalent is employed.

Aluminum, as suggested above, is normally supplied in papermakingprocess as alum, by which is meant bydrated aluminum sulfate. Otherwater-soluble aluminum salts can be employed in equivalent amounts, forexample, aluminum chloride, other alums, or sodium aluminate to providethe same concentration of alumina.

The term alum herein is used generically to mean aluminum sulfatehydrated with about eighteen molecules of water or its equivalent inaluminum content of another water-soluble aluminum compound.

Calcium chloride or other such water-soluble polyvalent metal salt canbe used in place of up to about half of the alum. Usually, severalpounds of calcium chloride are required to replace a pound of alum andstill provide the same results.

In the process of the invention, the polymer and the alum can bedissolved in the pulp suspension at any appropriate point prior toformation of the paper web. They are preferably added as dilute watersolutions. In a system where sodium aluminate is used as the source ofaluminum, a combination of the aluminate and the polymer inpre-determined proportions may be dissolved in water and added togetheror a dry blend of the two materials can also be used. When a sodiumaluminate-polymer combination is employed, it is important that the acidgroups in the polymer be in water-soluble salt form, usually the sodiumsalt, and that the polymer be dissolved in the presence of sodiumaluminate. Advantageously, such polymer-sodium aluminate blends containabout 1 to 4 parts by weight of anionic polymeric polyelectrolyte foreach 1 to about 30 parts of aluminate. Preferably, compositions of thiskind contain one part of polymer to from 1 to about 20 parts ofaluminate.

The polymers used in the following examples had average molecularweights of at least one million, generally in the range of 5-50 millionas calculated from intrinsic viscosity measurements. Usually, both thepolymers and the aluminum compound were added to the pulp suspensions asdescribed in the form of dilute aqueous solutions, but dry materialswere also used.

Example 1 A simulated kraft linerboard pulp system was made up bybeating 23 liters of 1.5 percent unbleached kraft pulp suspension with alaboratory beater until a Canadian Standard Freeness of about 400 ml.was reached. This was diluted to one percent fiber content with waterand 10% of dissolved black liquor solids (solid lignin residues) basedon the weight of fiber was added. From the resulting suspension, 300 ml.samples were withdrawn, diluted to 1000 ml. with water, and the pH wasadjusted to the desired level by adding 1 N HCl. Alum was added in apre-determined amount, the suspension was stirred for two minutes, andthe pH was then re-adjusted to its original level by appropriateaddition of 1 N NaOH. The desired amount of anionic polymer was mixedwith the suspension (as a 0.05% aqueous polymer solution) and thesuspension was poured into a vacuum filtration apparatus on which aconstant vacuum of 500 mm. of mercury was maintained. The time inseconds required to obtain 900 ml. of filtrate was then noted.

In this experiment, filtrations were run as described above at pH 5, 6and 7 using various quantities of alum and 0.05% based on the weight offiber of a copolymer of acrylamide and acrylic acid or correspondingpartially hydrolyzed polyacrylamide having an average molecular weightof the order of five million and containing from ten to ninety molepercent of acrylic acid moieties, sodium salt form, in its molecularstructure. Results are listed in Tables 1-3.

TABLE 2 Mole percent acrylic acid in polymer- Percent alum 10 30 50 7090 N o'rE.--Coutrols: No polymer, no alu.m160 seconds; no polymer 1%alum-170 seconds; no polymer, 3.75% alum190 seconds.

TABLE 3 Mole percent acrylic acid in polymer- Percent alum 10 30 50 70Norn.-Controls: No polymer, no alum-150 seconds.

Example 2 A pulp suspension was made up as described in Example 1 with0.3% fiber content, 10% black liquor solids, and a constant 3% alumloading. Vacuum filtration tests were run as in the above example afterthe pH was adjusted to the desired value and 0.05% of acrylicacidacrylamide copolymer of defined composition as described in Example1 was added. The filtration times in seconds required to collect 900 ml.of filtrate are listed in the table. Control runs with no polymer addedwere made for comparison.

FILTRATION TIME, SECONDS Percent alum (no polymer) (no polymer) EXAMPLE4 YA kraft linerboard pulp suspension was prepared as described inExample 1 to contain 0.3 percent pulp fiber and ten percent black liquorsolids based on the fiber content. The suspension was adjusted to thedesired pH value and alum and 0.05% polymer were added as previouslydescribed. In this case, the anionic polymer was sodiumpolystyrenesulfonate having an average molecular weight of about 5-10million. Filtration times for 900 ml. of filtrate were determined underconditions previously described.

Percent Percent Time, polymer alum pH seconds EXAMPLE 5 In the mannershown in Example 1, a kraft linerboard suspension was preparedcontaining 10% black liquor solids based on the 0.3% fiber content and0.05 of the ploymer described in Example 3. Experiments were performedwith various quantities of alum and combinations of alum and calciumchloride at pH 6.0. Calcium chloride was added after the pH was adjustedfollowing addition of alum. Filtration times of 900 ml. filtrate weredetermined as before.

Mole percent acrylic acid moieties in polymer Example 3 tration PercentPercent time, Following the procedure of Example 1, a kraft pulp 65 alumCaCh seconds suspension was made up to pH 6.2 and filtrations were 0 n9run as before to show the effect of black liquor SOlIdS 108concentration at different alum loadings. A partially hyo 351 drolyzedpolyacrylamide of about 5 million average mo- 3 lecular weight having20-25 percent of the initial car- 70 0 78 boxamide groups hydrolyzed tosodium carboxylate L7 1 19 groups was used at a level of 0.05concentration based L7 84 on the fiber content. Times listed are inseconds requlred a? to collect the total filtrate from a liter sample at500 mm 75 i No polymer was present.

Hg vacuum.

7 EXAMPLE 6 A typical kraft linerboard pulp suspension had the followingcomposition:

Based on fiber.

Four 550 ml. samples of the suspension were diluted to one liter withwater, the pH of each was adjusted to one liter with water, the pH ofeach was adjusted with NaOH or HCl as desired, anda dry, blended mixtureof five parts by weight sodium aluminate to one part of the partiallyhydrolyzed acrylamide polymer of Example. 3 was added to two of thesuspension samples in a quantity to provide 0.05% of polymer based onthe fiber content. The samples were then suction filtered using 20inches Hg vacuum and the times in seconds required to obtain a measuredquantity of filtrate was noted in each case. The results are summarizedin the following table.

Time for volume of filtrate, 1111. Final Treatment pH 500 700 900Control 7.0 16 34 60 6.0 16 32 65 P01 er luSNaAlOz 7.0 7 13 23 ym p 6.019 34 We claim:

substituted for a portion of the aluminum compound in an amount of up toabout half of the aluminum compound.

4. The process of claim 1 wherein the polymer is one in which themolecular structure is composed essentially of carboxyalkylene units ofthe formula and carbamoylalkylene units of the formula in whichstructure the carboxyalkylene units are at least about 25 percent of thetotal units present and in which each R is hydrogen or a methyl radicaland M is hydrogen or a water-soluble salt-forming cation.

5. The process of claim 4 wherein the polymer is added to the pulpsuspension in combination with sodium aluminate.

6. The process of claim 4 wherein the pulp suspension is an unbleachedkraft liner-board suspension containing about 2-10 percent of ligninresidues and the suspension has a pH of about 5.5-7.5.

7. The process of claim 6 wherein R is hydrogen in the carboxyalkyleneand carbamoylalkylene units of the polymer structure.

8. The process of claim 7 wherein the water-soluble aluminum compound isalum and the alum is present in a quantity of about 02-10 percent basedon the weight of cellulosic pulp.

9. The process of claim 8 wherein about 1-5 percent of alum is present.

10. The process of claim 4 wherein the pulp suspension is a neutralsulfite semi-chemical pulp suspension containing about 25-100 percent oflignin residues and the suspension has a pH of about 7-9. p

11. The process of claim 10 wherein R is hydrogen in the carboxyalkyleneand carbamoylalkylene units of the polymer structure.

12. The process of claim 10 wherein the water-soluble aluminum compoundis alum and the alum is present in a quantity of about 0.1-10 percentbased on the weight of cellulosic pulp.

13. The process of claim 10 wherein about l-5 percent of alum ispresent.

References Cited UNITED STATES PATENTS 2,972,560 2/1961 Stilbert et al162-l68 3,019,157 1/ 1962 Reynolds et al. 162168 3,281,312 10/ 1966Fetters et al. 162-168 3,222,245 12/1965 Poschmann et al. 162-1683,258,393 6/ 1966 Woodberry et al. 162-163 X 3,305,435 2/ 1967 Willistonet a1. 162163 X OTHER REFERENCES Casey: Pulp & Paper, vol. 2, 1960, p.978, Interscience, N.Y.

S. LEON BASHORE, Primary Examiner F. FREI, Assistant Examiner US. Cl.C.X.

